* Any rescue attempt fraught with problems * Rebel video is little indication of girls' location * Some soldiers say morale for fight with rebels low ABUJA, May 15 - Nigeria's hunt for more than 200 abducted schoolgirls is not all that it seems. In public, an international operation is gathering pace while behind the scenes, officials say it is unlikely to deliver the success that global opinion demands. The United States and Britain are helping Nigerian forces in the effort to liberate the girls taken from their school in Borno state a month ago by Boko Haram Islamist militants. The pressure for results is huge, with the likes of Michelle Obama and film star Angelina Jolie supporting a social media campaign operating under the Twitter tag #BringBackOurGirls. Washington has sent surveillance aircraft as well as assigning medical, intelligence, counter-terrorism and communications advisers to the mission. But officials have little idea where the girls are, and acknowledge that if they are found, any rescue attempt would be fraught with problems. On top of that, morale is shaky among some of the Nigerian troops involved in the hunt who already have experience of Boko Haram as a formidable foe. "We commend the effort by the #BringBackOurGirls protesters but it doesn't fit with the reality of the security situation we are facing," said a senior Nigerian military source who spoke on condition of anonymity. Foreign experts are also pessimistic that the girls can be extricated from the rebels' clutches and returned to their homes in Nigeria's remote northeast where Boko Haram operates. "I think a rescue is currently unlikely and unfeasible," said Jacob Zenn, a Boko Haram expert at U.S. counter-terrorism institution CTC Sentinel. Until Monday, nothing had been seen of the girls since they were snatched from the village of Chibok near Nigeria's borders with Cameroon, Chad and Niger. Then Boko Haram released a video showing more than 100 girls together in a rural location. In it, rebel leader Abubakar Shekau offered to exchange them for captured militants. The video raised hopes that their location could be found using ground forces, state-of-the-art intelligence and surveillance planes. Then an operation could be staged, perhaps with forces swooping from the sky like a British raid in Sierra Leone in 2000 to free soldiers held by militiamen, or Israeli commandoes' rescue of passengers from a jet hijacked to Entebbe, Uganda, in 1976. However, such a scenario is unlikely this time. One source with knowledge of the search said the footage was probably taken at least 10 days ago, if Boko Harma's past videos are any guide. By now, the girls could be somewhere else as a group, or dispersed to many places.Virtualy undetectable The Sambisa forest, Boko Haram's stronghold, is a first target but it is not conducive to aerial search because it covers 60,000 square km (23,000 square miles), more than twice the size of Rwanda. The rebels know this area intimately and could spread the girls among local families, making them virtually undetectable by conventional security forces. Two U.S. national security sources said initially the girls were separated into around three large groups but were subsequently scattered in smaller groups. Other experts said they could be in mountains near Gwoza on the Cameroon border. If this is the case, some girls might be found before others, posing a dilemma for would-be rescuers. "In the past, Boko have threatened, and maybe actually gone ahead with, killing hostages upon sensing the merest hint of possible rescue operations," said a security source. Rescuing some girls could add to danger faced by others still captive. Another problem is time. Britain's minister responsible for African affairs, Mark Simmonds, said on Wednesday that it was "early days" in the rescue operation, yet the abduction happened on April 14 so rebels have had ample time to prepare for an international response. A senior U.S. Defense Department official criticised Nigeria on Thursday for being too slow to adapt to the threat of Boko Haram.Tired of fighting An incident this week highlighted the problems of the Nigerian army, whose 1st and 7th divisions have been deployed to Maiduguri, the capital of Borno state, to confront the rebels. On Tuesday, gunmen ambushed a unit returning to Maiduguri from Chibok, killing at least four soldiers. Rebels were also killed. The next day soldiers fired into the air at Maiduguri barracks to protest about what they said was poor leadership. Defence headquarters quickly stated that the situation had been resolved, but soldiers told Reuters they remained angry. "These Boko Haram have special forces. They know how to shoot and maintain their grounds more than us. What is making it worse is our superior officers who are insensitive to the plight of the troops," said a soldier, who declined to be identified. "We are tired of this whole thing because we are being killed every day and we don't get the required support and care from our superiors," another soldier said. President Goodluck Jonathan will visit Chibok on Friday, senior government officials said. Even coordinating an international effort faces difficulties in Nigeria, which recently overtook South Africa as the continent's biggest economy. Nigeria has close ties with Western powers but has historically resisted foreign military involvement on its soil. One possible sign of differing approaches is that Simmonds, rather than the president himself, announced that Jonathan had ruled out any prisoner exchange for the girls' release. Nigerian officials have since declined to comment. Ultimately, the girls' best hope may lie in dialogue but the road to talks remains uncertain because the rebels do not form a unified group. Boko Haram is faceless and even Shekau heads just one of several loosely coordinated groups with differing objectives, said a senior official with knowledge of the northeast. A Nigerian presidential committee set up last year for talks with the rebels dealt last year with Boko Haram proxies. But they were later denounced by other Boko Haram militants as impostors, according to Minister of Special Duties Tanimu Turaki, who leads the committee. One security source in Abuja cautioned against raising false hopes. "It is time we removed the thought of a very happy ending to this situation," the source said.